1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor carrying a radiation image of an object such as the human body stored therein to stimulating rays which cause the stimulable phosphor to emit light in proportion to the radiation energy stored, guiding the emitted light to a photodetector via a light guide member, and obtaining an electric signal representing the radiation image.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed, for example, in U.S. Pat. No. 4,239,968, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image thereof stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image of the object on a recording medium such as a photographic light-sensitive material or on a display device such as a cathode ray tube (CRT).
In the aforesaid radiation image recording and reproducing system, the energy of the light emitted by the stimulable phosphor sheet when it is exposed to stimulating rays is very much smaller than the energy of stimulating rays (approximately 10.sup.-5 times the energy of stimulating rays), and the emitted light is non-directional. Therefore, when the read-out apparatus is fabricated, the light receiving solid angle of the light guide member should be made as large as possible to guide as much emitted light as possible and improve the S/N ratio.
Accordingly, the applicant proposed in U.S. Pat. No. 4,302,671 to provide the light receiving face of the photodetector, which is positioned close to a stimulable phosphor sheet in face-to-face relation, with a reflecting mirror or a triangular prism sufficiently smaller than the area of the light receiving face of the photodetector so that the reflecting mirror or the triangular prism protrudes from the light receiving face. In this technique, stimulating rays such as a laser beam incident laterally and exhibiting high directivity are reflected by the reflecting mirror or the triangular prism onto the stimulable phosphor sheet. However, in this case, a space at least equal to the amount of protrusion of the reflecting mirror or the triangular prism must be left between the light receiving face and the stimulable phosphor sheet. Therefore, the light receiving solid angle of the light guide member becomes small.
The applicant also proposed in U.S. Pat. No. 4,346,295 to position a light guide member having a special shape so that one end face (light input face) is close to a stimulable phosphor sheet in face-to-face relation thereto along a scanning line of stimulating rays, and to guide light emitted by the stimulable phosphor sheet upon stimulation thereof to a photodetector by use of the light guide member. However, in this case, since it is necessary to prevent interference between the stimulating rays and the light guide member, it is impossible to cover the whole area above the scanning line by the light input face of the light guide member and to make the light receiving solid angle sufficiently large.
Therefore, in aforesaid U.S. Pat. No. 4,346,295, the applicant further proposed to position a light reflecting means standing face to face with the light input face of the light guide member so that light emitted by the stimulable phosphor sheet in the direction opposite to the light input face is reflected by the light reflecting means and guided to the light input face. However, even in this technique, a space for passing the stimulating rays must be left between the light input face of the light guide member and the light reflecting means, and the problem that a part of the light emitted by the stimulable phosphor sheet is lost is not solved.
As described above, in the conventional radiation image read-out apparatuses, since a part of the light emitted by the stimulable phosphor sheet is lost without being detected, the amount of the stimulating rays must be made large for increasing the amount of light emitted by the stimulable phosphor sheet, and the stimulating ray source becomes large. Also, for the same reason, the scanning speed is limited.
Therefore, in order to provide a radiation image read-out apparatus wherein the solid angle for receiving light emitted by a stimulable phosphor sheet upon stimulation thereof is adjusted sufficiently large, and nearly all emitted light is received and detected, and wherein the S/N ratio is improved, the stimulating ray source is made small, and the scanning speed is increased, the applicant proposed a novel radiation image read-out apparatus in Japanese Patent Application No. 58(1983)-218678 whose U.S. equivalent is Ser. No. 672,683, now U.S. Pat. No. 4,564,760. In this apparatus, a dichroic optical plane selectively reflecting either one of stimulating rays and light emitted by a stimulable phosphor and selectively passing the other is positioned in an optical path of a light guide member. Further, a stimulating ray optical system is positioned so that stimulating rays directly impinge upon the dichroic optical plane and come out of one end face of the light guide member positioned close to the stimulable phosphor by being reflected by the dichroic optical plane or by passing therethrough, thereby stimulating the stimulable phosphor.
However, it was found that a problem as described below still arises in the radiation image read-out apparatus employing the dichroic optical plane. That is, stimulating rays reflected by the surface of a stimulable phosphor sheet are further reflected by the light input face of the light guide member onto other poortions of the stimulable phosphor sheet that should not be exposed to the stimulating rays in the exposure step at that time. As a result, light is emitted also by the other portions of the stimulable phosphor sheet and enters the light guide member. Such light constitutes noise and adversely affects the sharpness and contrast of the reproduced image.